So, with a tankless, you are heating with combustible gases in a heat exchanger manifold. My thought is that steam would be MUCH more efficient depending on the quality of the steam. The specific enthalpy of steam is much higher than gas and I would think your efficiency would be better.
I still wouldn't get away from the HLT idea though. You still have the ability to adjust your temp in the HLT once it comes out of the heat exchanger. Also, capturing your cooling water for cleaning is a big benefit